Ugh, it looks like the ouija board has become the newest victim of science's ceaseless vendetta against my childhood ambition of growing up to be a very successful witch. It seems that science won't be satiated until it has me drinking a bitter potion made of tears shed whilst watching the Long Island Medium, which is the magical equivalent of the idiom "eating humble pie."
According to the BBC, you and your little slumber party buddies never actually conversed with Methuselah or Czar Nicolas III or Harriet Tubman or whoever it is that kids are trying to contact via ouija board these days — and, what's even worse, James Franco never managed to get his claws into the spirit world to summon Tennessee Williams. This is bad news for everyone.
The movements of the ouija board cup are really caused by a phenomenon known as the ideomotor effect, wherein the subject makes motions unconsciously. These small, unconscious motions can cause movement that seems to come from a supernatural source — this is also true for other mystical practices, such as dowsing. The ideomotor effect is not magic in the traditional understanding, says the BBC; rather it's "the ordinary everyday magic of consciousness," which, as everyone knows, is the most boring and worst magic of all. There is no Higginbottoms School of the Ordinary Everyday Magic of Consciousness for a reason.
What's interesting about this phenomenon, though (aside from the way it crushes our dreams), is the way it complicates our understanding of "owning" an action:

The interesting thing about the phenomenon is what it says about the mind. That we can make movements that we don't realise we're making suggests that we shouldn't be so confident in our other judgements about what movements we think are ours.
Some psychologists argue that the ideomotor effect indicates that consciousness is less like a commanding general that orders your body to move about and more like part of an organized collective that affects movement and decision-making. According to a theory formulated by psychologist Daniel Wegner:

[O]ur normal sense of owning an action is an illusion, or — if you will — a construction. The mental processes which directly control our movements are not connected to the same processes which figure out what caused what.